The term “MDI” refers to a large number of technically important, but also structurally different, isocyanates. They include both monomers, in which two aromatic structural elements are bonded with one another via a methylene bridge (monomeric MDI), and also higher oligomers, in which more than two aromatic structural elements are bonded with one another in succession via a plurality of methylene bridges (polymeric MDI).
From the user's point of view, it is of great interest to obtain, where possible, the 4,4′- and the 2,4-isomer or mixtures of those two isomers.
The ratio of monomeric to polymeric MDI, and also the proportions of the 4,4′- and 2,4′-isomers in monomeric MDI, can be varied within wide limits by varying the synthesis conditions for the preparation of the precursor.
The separation of the crude MDI obtained in MDI synthesis is for the most part carried out by distillation, wherein there can be separated off, depending on the technical outlay, either almost isomerically pure fractions with contents of 4,4′-MDI, for example, of greater than 97.5 wt. %, or isomeric mixtures with contents of 4,4′- and 2,4′-MDI of in each case about 50 wt. %.
Very recently there has additionally been an increasing need for the 2,4′-isomer. This is due substantially to the different reactivities of the NCO groups in the 2- and 4-positions of 2,4′-MDI (similar to the differences in reactivity of the NCO groups in the 2- and 4-positions of 2,4-toluoylene diisocyanate (TDI)).
These differences in reactivity permit or facilitate the synthesis of low-monomer NCO prepolymers (NCO prepolymers are intermediates which can be isolated during the preparation of finished end polymers—they carry unchanged NCO groups at their chain ends. These are obtained by reacting a polyol with a molar excess of an isocyanate (based on the NCO-reactive or NCO groups) at from room temperature to about 100° C.).
In the case of asymmetric isocyanates (=isocyanates having at least two NCO groups of different reactivities), preferably only one NCO group reacts with the polyol, while the other remains unchanged. The prepolymerization is accordingly considerably simpler to control than is the case with most NCO prepolymers of the prior art, in which there are used isocyanates whose NCO groups do not have different reactivities. In comparison therewith, these also always contain larger amounts of free, monomeric diisocyanate.
The preparation of low-monomer to virtually monomer-free NCO prepolymers is highly desirable in the case of 2,4-TDI because it has a high vapor pressure and polymers containing unreacted 2,4-TDI constitute a high health risk. The NCO prepolymers based on aliphatic diisocyanates, such as, for example, hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI), are to be regarded as even more critical in this respect.
This aspect is also to be taken into consideration in the case of MDI, but it comes to bear to only a reduced extent because of its lower vapor pressure as compared with TDI.
TDI or low-monomer TDI prepolymers are still the current state of the art; the use of pure 2,4′-MDI-containing prepolymers is relatively new on the market.
The preparation of low-monomer NCO prepolymers can be carried out in several ways:                1. By removal of the free monomeric diisocyanate by technically complex thin-layer or short-path evaporation. This possibility can be used regardless of whether diisocyanates having NCO groups of the same or different reactivities are used. Entrainers, for example, can also be used therefor.        2. By the use of diisocyanates having NCO groups of different reactivities or NCO groups of the same reactivity and specially selected stoichiometric ratios, for example molar ratios of NCO to NCO-reactive groups of below 2:1, and/or optionally with special catalysis.        3. Combinations of the two processes, for example in such a manner that the content of free monomeric diisocyanate according to process 2 is first limited to a specific degree and is then minimised further by process 1.        
Such combinations can be expedient when the viscosity of the prepolymers is to be minimised. The disadvantage of process 2 is, in principle, that reactions with stoichiometric ratios, in particular below 2:1, lead to increased pre-extension, and there is accordingly an inherent marked increase in the viscosity of the reaction product.
International Pat. Pub. No. WO 01/40340 A2, the entire contents of which are hereby incorporated herein by reference, gives examples of such combinations of process steps, wherein in a first stage the diisocyanate is reacted, using a selectivity-increasing catalyst, to give a NCO prepolymer, which is then freed of excess monomer by thin-layer evaporation.
For particularly critical applications, such as, for example, in the foodstuffs sector, particularly high demands are made of a residual monomer content (key word work hygiene). This is true to a large degree for TDI, but in some cases also for MDI (see above). An indication therefor are numerous publications or patents which are also concerned with low-monomer MDI prepolymers, for example International Pat. Pub. Nos. WO 03/006521, WO 03/033562, WO 03/055929, WO 03/051951, WO 93/09158 and European Pat. Pub. No. EP 0 693 511 A1, the entire contents of each of which are hereby incorporated herein by reference.
For the above-mentioned reasons, the need for the 2,4′-MDI isomer has increased recently, as mentioned. However, owing to the process, this in principle also results in an increased amount of 2,2′-MDI, which has to date been regarded as unusable. For example, it is stated in International Pat. Pub. No. WO 2007/087987, the entire contents of which are hereby incorporated herein by reference: “In the case of monomeric MDI, the 4,4′- and 2,4′-isomers are predominant, owing to the synthesis. The 2,2′-isomer, which is less frequent and is largely of no commercial value, is also formed to a lesser degree”.
This is partly because pure 2,2′-MDI is not available industrially and 2,2′-containing formulations often have the disadvantage that this monomer reacts markedly more slowly and therefore less completely. This can lead, for example, to undesirable migration during the bonding of foodstuffs packaging or to the blowing off of foams.
2,2′-MDI, or the formulations containing it, is therefore regarded as waste and must be disposed of in an expensive manner.
Alternative disposal or use of 2,2′-MDI mentioned in the prior art includes the possibility of using 2,2′-MDI for controlling the reaction velocity of isocyanate mixtures containing it. The following possibilities, inter alia, are described in the prior art for controlling the reaction velocity of MDI mixtures:                1. Mixtures with TDI or TDI prepolymers with the associated disadvantage of increased toxicity. Owing to the higher vapor pressure of the TDI monomer, even permitted residual contents of 0.5% lead to disturbing odors and impairments.        2. Extenders in the form of plasticizers, or hydrocarbons in general. There are used, for example, phthalates such as DINP or alternative plasticizers such as 2-cyclohexane-dicarboxylic acid diisononyl ester, acetyltributyl citrate or solvents such as ethylene/propylene carbonate, dibasic esters (DBE), solvent naphtha and benzines. Disadvantages here are primarily a reduction in quality and the risk of migration of the non-chemically-bonded additives and the accompanying changes in properties over time.        3. High contents of 2,4-MDI (>85%), see WO 2007/087987 with the accompanying high outlay in terms of isomer separation and the associated cost disadvantages.        